Angiogenesis, the generation of capillaries, is virtually absent in healthy adult organism and is restricted to a few conditions including wound healing and the formation of corpus luteum, endometrium and placenta. The endothelial cell plays a key role in the formation of new capillaries, and the neovascularization process occurs via a series of sequential steps, which are similar regardless of the nature of the inducing stimulus. These steps can be summarized as follows: the endothelial cells that line existing microvessels focally degrade the basement membrane through a finely-tuned elaboration of proteolytic enzymes and their inhibitors, and form tiny sprouts which invade the perivascular connective tissue. As these sprouts elongate by migration of endothelial cells below the tip, a luman is gradually formed. The hollow sprouts thus generated anastomose with each other to form capillary loops through which blood begins to flow. New sprouts then arise from each loop and eventually give rise to an entire capillary network.
Under normal physiological conditions in the healthy adult mammal, during cyclical changes in the female reproductive tract or in response to wounding, the coordinated sequential cellular events leading to new capillaries are spatially and temporally restricted so that the disturbed balance between naturally occurring inducers and inhibitors of neovascularization rapidly reverts to the normal situation, in which inhibitory influences predominate.
In contrast thereto, in certain pathological conditions angiogenesis is dramatically enhanced and is no longer self-limited, i.e. a result of well-balanced activity of angiogenesis inhibitors and stimulators. Pathological angiogenesis is seen during the development and progression of many diseases, such as in rheumatoid arthritis, psoriasis and diabetic retinopathy. These diseases were recently called angiogenic diseases and share at least two characteristics in common: 1) an abnormality of capillary blood vessel growth is the principal pathological feature and 2) the possibility that therapeutic control of the abnormal capillary growth would ameliorate or eliminate other manifestations of the disease. Probably the clinically most important manifestation of pathological angiogenesis is that induced by solid tumors [Folkman, J. (1985) Adv. Cancer Res. 43, 175-203 and Nature Medicine, vol. 1, number 1, (1995)]. If a neoplasm is to grow progressively as a solid mass consisting of layers of living cells more than a few millimeters thick, it must induce nearby capillaries to sprout and develop a new vascular network around and within the tumor. The new vascular network supplies the tumor with vital nutrients and oxygen and provides a removal route for toxic products of the active cell metabolism. Furthermore, new tumor vessels provide a port of exit for tumor cells to metastize to distant sites. Thus, the progressive growth of a solid tumor to develop into a life-threatening malignancy is strictly dependent on angiogenesis.
Estradiol, the major ovarian estrogen, has been extensively studied with regard to its physiological and pharmacological effects, and its involvement in endocrine-dependent carcinogenesis. On the contrary, very little is known about the contribution to the total estrogen expression of the more than 20 estrogen metabolites which are present in the female organism [Fotsis, T., Adlercreutz, H., J. Steroid Biochem., 28, 203 (1987); Fotsis T., Steroid. Biochem. 28, 215 (1987)]. Even less is known about their pharmacological effects and almost nothing about their effects on endothelial cells and angiogenesis.